The simplest way to preserve the
unitarity is to admit the existence of a ghost. But, as we described
above, this leads to the physically inconsistent output. Thus, one
avoids ghosts by forming the in states only with gravitons.
Owing to the interactions, ghost wakes up from the vacuum and
emerge in the out states – that means the scattering matrix
is not unitary [1]} and we arrive at the contradiction.
